1. Field of the Invention
This invention relates to a universal serial bus (USB) environment, and more particularly to a method and apparatus to improve an enhanced host controller interface (EHCI) performance for USB devices.
2. Description of the Related Art
In many of today's processors and systems, such as personal computer (PC) systems, there exist USB ports for connecting various USB devices. Many USB devices are frequently used by PC users. For example, USB devices may be printers, compact disc read-only memory (CD-ROM) drives, CD-ROM writer (CDRW) drives, digital versatile disc (DVD) drives, cameras, pointing devices (e.g., computer mouse), keyboards, joy-sticks, hard-drives, speakers, etc.
Different standards of USB technology have different bandwidths. For example, Universal Serial Bus Specification, revision 1.1, Sep. 23, 1998 (USB 1.1) devices are capable of operating at 12 Mbits/second (Mbps), and Universal Serial Bus Specification, revision 2.0, Apr. 27, 2000 (USB 2.0; also known as high-speed USB) devices are capable of operating at 480 Mbps. USB 2.0 defines a multiple speed-signaling environment where a single high-speed bus may support one or more USB 1.1 classic busses through a USB 2.0 hub (Transaction Translator). In this environment, system software (the Host Controller Driver) must allocate and manage the bandwidth of USB 1.1 classic busses.
The Enhanced Host Controller Interface (EHCI) specification for a Universal Serial Bus, revision 0.95, Nov. 10, 2000 describes the register-level interface for a host controller (HC) for USB 2.0. The USB 2.0 HC is a bus master on the peripheral component interconnect (PCI) bus. The HC independently traverses linked memory structures created and maintained by the HC driver to initiate transactions on the USB. In the USB EHCI specification, two data structures known as the queue head (QH) and the queue element transfer descriptor (qTD) are defined.
The QH contains all of the endpoint specific information required. The QH also contains links to a list of qTDs. A qTD represents all or part of a buffer passed to the HC driver from a higher level driver, or user level application. The QH contains all of the endpoint specific information required. The QH also contains links to a list of qTDs. A qTD represents all or part of a buffer passed to the HC driver from a higher level driver, or user level application.
The qTD data structure is only used with a QH. The qTD is used for one or more USB transactions. A qTD can, at most, transfer 20,480 (5*4,096) bytes. When the HC completes execution of a qTD, it will follow the next-pointer. When a USB device returns less data than requested by the qTD, a short packet condition results. When the HC receives a short packet, the HC follows the alternate-pointer.